1. Field of the Invention
The present invention relates to a carriage servo control system and an information-recording medium in which a program for carriage servo control is recorded. More particularly, the present invention relates to a carriage servo control system for servo-controlling movement of a carriage that comprises a pickup operative to optically record/reproduce information on/from a recording medium such as an optical disc, and to an information-recording medium in which the carriage servo control program for controlling the movement of the carriage is recorded.
2. Description of the Related Art
Generally, when recording/reproducing information on/from an optical disc, such as a CD (compact disc), a DVD (digital versatile disc; which is an optical disc having a recording capacity that is several times larger than a conventional compact disc), by transmitting an optical beam to the optical disc, it is necessary to carry out various types of servo control to exactly record/reproduce information on/from the optical disc.
More specifically, the various types of servo control include a focus servo control, a tracking servo control and a spindle servo control. Of these, the focus servo control is executed to cancel an error between a focus position of an optical beam and the position of a target track now under use for recording/reproducing the information on/from the optical disc. In this case, the error is detected in the direction vertical to the information record surface of the optical disc. The tracking servo control is executed to cancel an error between a focus position of an optical beam and the position of a target track in a direction parallel to the information record surface, that is, the radial direction of the optical disc. The spindle servo control is executed to exactly rotate the optical disc.
Both of the focus servo control and the tracking servo control are carried out by individually moving an object lens which focuses an optical beam on a target track in the vertical direction to the information record surface or the parallel direction parallel thereto. In order to make progress a recording/reproducing operation under the rotation of the optical disc on which the tracks are spirally formed, it is necessary to move the beam's focus position (radiated position) beyond a movable range of the object lens in the parallel direction. In this case, it is further necessary to move in the radial direction the whole pickup unit including a semiconductor laser for generating the optical beam, the objective lens, a photo acceptance unit for accepting the reflected beam of the optical beam, and others. That is, carriage servo control must be needed.
One type of the conventional carriage servo control is explained with reference to FIGS. 7(A) and 7(B). FIG. 7(A) is a block diagram showing a carriage serve control system for executing one mode of the conventional carriage servo control, while FIG. 7(B) is an operating waveform diagram of the conventional carriage servo control system.
As shown in FIG. 7(A), in the conventional carriage servo control system, a tracking error signal TE showing the error between an optical beam's radiated position and a target track position in the radial direction is used. Concretely, the tracking error signal TE is produced on the basis of a reflected beam from the information-recorded surface using a technique, such as a three-beam method or a heterodyne method.
The conventional carriage servo control system usually comprises a tracking equalizer 3, a carriage equalizer 19, a comparator 5, a switch 4, a driver 7c and a carriage motor 8, all of which operate as follows. The tracking equalizer 3 executes phase compensation on the produced tracking error signal TE. The carriage equalizer 19 executes gain adjustment as well as phase compensation on an output signal A from the tracking equalizer 3 for the carriage servo control. The comparator 5 compares an output signal B from the carriage equalizer 19 with a reference voltage Vz. The switch 4 operates to turn the output signal B from the carriage equalizer 19 on and off according to a signal supplied from the comparator as a switch control signal. Further, the driver 7c amplifies in current an output signal C coming from the switch 4. Still further, the carriage motor 8 rotationally drives a shaft 12 connected to a carriage (not shown) on which the pickup is mounted so that the carriage is movable together with the pickup in the radial direction.
The operation of the conventional carriage servo control system will now be explained.
First, while the tracking servo control is executed (that is, a beam's radiated position and a target track position in the radial direction agree with each other), the optical disc is rotated while being subjected to the spindle control.
Under such a condition, a not shown tracking coil, which is configured to move the objective lens in the radial direction, is driven so that the objective lens follows the spiral track in the radial direction.
In order to move the objective lens beyond its movable range in the radial direction, it is necessary to drive the carriage motor 8 to shift the pickup by only several tracks in the radial direction. The output signal A shown at the top in FIG. 7(B) is fluctuated in response to fluctuations in the tracking error signal TE, which are caused due to movements of both the objective lens and the pickup. That is, both of the tracking servo control and the carriage servo control cause the tracking error signal TE (output signal A) to be linearly changed, with pulsated, responsively to movements of the objective lens in the radial direction. The pulsation of the signal TE results from eccentric motions of the optical disc. When the objective lens reaches a limit position of its movable range, the sign of the tracking error signal TE is reversed by the movement of the whole pickup in the radial direction (the movement thereof on the basis of the carriage servo control). Thus, the linear change of the tracking error signal TE is started again.
The carriage equalizer 19 passes low frequency components of the output signal A, so that the output signal A is converted into the output signal B suitable for the carriage motor 8 (refer to the second from the top in FIG. 7(B)).
Because the tracking error signal TE gradually changes as the objective lens follows up the spiral track, the output signal B exceeds a standard voltage Vz in the comparator 5 at a certain time. The foregoing switch control signal is thus supplied to the switch 4.
The output signal B from the carriage equalizer 19 is turned on or off by the switch 4 responding to the switch control signal. Hence the output signal C with the waveform shown at the bottom in FIG. 7(B) is supplied to the driver 7c as a drive signal. The drive signal is amplified by the driver 7c, and then supplied to the carriage motor 8.
Likewise, a series of operations stated above are repeated. Therefore, whenever the lower frequency component of the tracking error signal TE exceeds the reference voltage Vz due to the fact that the position of the objective lens inside the pickup is moved by the tracking servo control, the carriage motor 8 is intermittently driven.
However, there is a problem that the operation of the conventional carriage servo control system is still unsatisfactory in terms of a stable servo operation. To be specific, the carriage servo control system is greatly affected by fluctuations in voltage on account of eccentric motions of the optical disc (the pulsated components shown in FIG. 7(B)). This affection results in an unstable servo operation.
Moreover, because the carriage motor 8 is driven by the drive signal including the fluctuated components corresponding to the eccentric motions of the optical disc, the operations of the carriage motor 8 themselves are unstable. In other words, in FIG. 7(B), when the output signal B on which the fluctuated components are overlapped is outputted from the carriage equalizer 19 (refer to the second from the top in FIG. 7(B)), the fluctuated components are directly reflected in the drive signal (output signal C). The drive power supplied to the carriage motor 8 corresponds to an integral value of the drive signal, which is shown by hatched portions in the bottom in FIG. 7(B). Thus, the fluctuated components corresponding to the eccentric motions of the optical disc are included in the drive signal, the integral values are also subject to changes, with the result that the drive signal of constant amplitude will not be supplied to the carriage motor 8 with a stable manner. In addition, there are some cases in which the drive signal is split into several signal parts, as shown by the first pulse at the bottom in FIG. 7(B). This split, if happens, also gives unstable operations to the carriage motor 8.
Furthermore, the integral values also change by irregularities in characteristics of the carriage equalizer 19 as well as those of the carriage motor 8.
The foregoing various types of fluctuated components (irregularities), therefore, cause the carriage servo control system not to operate precisely, differently from the designed specifications, so that unstable operations are still left in the carriage servo control system. Further, to suppress such irregularities requires the specifications of the carriage servo control system be redesigned. This remarkably lowers degrees of freedom in designing the whole carriage servo control system, thereby greatly narrowing applications of the carriage servo control system. Additionally, there occurs a problem that the redesign for suppressing the irregularities increases the number of steps of the design itself.